Microporous membranes are useful as battery separator film (“BSF”) for primary and secondary batteries. Such batteries include lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, etc.
It is desirable for a battery separator film to have a relatively low heat shrinkage, especially high temperature, in order to prevent internal short circuits. Microporous membranes having a heat shrinkage in the range of about 1.0% to 10.0% at 105° C. have been made using polyolefin. For example, JP60-242035A discloses a membrane made by a process comprising molding a gel-like sheet made by extruding a solution containing solvent and polyolefin having a weight average molecular weight≧7.0×105, removing the solvent from the gel-like sheet, and then stretching the gel-like sheet.
It is also desirable for microporous membranes to have relatively high air permeability, pin puncture strength, and meltdown temperature. For example, Japanese patent applications JP59-196706A and JP61-227804A disclose the use of polymethylpentene (PMP) to increase membrane meltdown temperature for improved battery safety. These membranes, however, have a relatively high shutdown temperature. Japanese patent applications JP07-060084A and JP3634397B disclose microporous membrane comprising polyethylene and polymethylpentene, the membrane being produced by mixing polyethylene and polymethylpentene with solvent or third polymer. The film is said to have a relatively low shutdown temperature and a relatively high meltdown temperature. Further, U.S. Pat. No. 6,096,213 discloses a method for making membrane comprising polyethylene and polymethylpentene without using any solvent or third polymer. Japanese patent application JP2004-161899A discloses a microporous membrane comprising polyethylene and polymethylpentene having a relatively high air permeability and a low heat shrinkage at 105° C. Japanese patent application JP2005-145999 discloses a microporous membrane comprising polymethylpentene and alpha-olefin copolymer. While polymethylpentene has been used to improve the properties of microporous membranes, further improvements are desired.